Touch screens have touch coordinate detection systems mounted at the front of displays (e.g., CRTs, LCDs). Many different types of touch detection systems based on different physical principles have been tried. Examples include touch screens based on optical, acoustical, and electronic technologies and there are numerous variations within each category. Some touch screen technologies use an analog/vector approach to locate touches and therefore do not localize touches on a predetermined grid. However, many types of touch screens localize touches using a fixed 2-D grid which can be based on optical or electrical impedance change sensing.
The category of touch panels that use a predetermined grid can be further sub-divided into two categories. One category is referred to herein as “M×N” (where M and N stand for integers and M×N is the product of those integers). Touch screens in the M×N category effectively divide the sensing area into M×N independent sensors, so that when a touch is detected by an M×N system, both of the coordinates (e.g., the X and Y coordinates) of the touch are determined at once because each individual sensor has a particular X coordinate and a particular Y coordinate. A drawback of some electrical M×N systems is that there are many individual sensors to be interrogated. The number of sensors to be interrogated implies a requirement for a high bandwidth data bus or a slow frame rate for sensing. For certain applications of touch screens, such as hand writing recognition, it is desirable to achieve a high rate of touch coordinate updating, and for such applications M×N systems present limitations.
Another category of touch panels that uses a predetermined grid is referred to herein as “M+N” (where M and N stand for integers and M+N is the sum of those integers). An M+N type touch panel separately detects the X coordinate of touches using one sub-system (e.g., including an array of vertically extending electrodes) and separately detects the Y coordinates using another sub-system (e.g., including an array of horizontally extending electrodes). Generally, for touch screens of practical interest, the integers M and N will have sufficiently high values such that M×N will greatly exceed M+N. Accordingly, an M+N system will require far lower data rates to achieve a certain touch coordinate update rate, and therefore applications that require high touch coordinate update rates such as hand writing recognition are more easily supported.
The above-mentioned separation of the detection of the X and Y coordinates presents no problem if only a single touch is to be detected, because the X and Y coordinates of the single touch are assumed to be correlated. However, in order to support more complicated touch screen interactions (e.g., gestures) it is desirable to be able to detect two or more touches contemporaneously. For example a user can touch a touch screen using their thumb and index finger and move their thumb and index finger along arcuate paths in order to input a rotation command which could then be interpreted to call for rotation of a displayed graphic (e.g., map), for example. In the case of an M+N system that detects the X and Y coordinates separately, two contemporaneous touches (i.e., a multi-touch) can confound the system because the system will be unable to unambiguously associate the two detected X coordinates with the two detected Y coordinates. Consequently, software applications that rely on the M+N touch detection system will be unable to determine if the user called for a clockwise rotation or a counter clockwise rotation, for example.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.